fix: precheck local downloads to avoid JSON file download on quota errors

frontend/app.js

@@ -1562,7 +1562,7 @@ handleDragLeave(e) {
       }
 
       // 其他场景走后端下载接口（支持下载流量计量/权限控制）
-      this.downloadFromLocal(filePath);
+      await this.downloadFromLocal(filePath);
     },
 
     async downloadFromOSS(filePath) {
@@ -1595,15 +1595,35 @@ handleDragLeave(e) {
       }
     },
 
-    // 本地存储下载
-    downloadFromLocal(filePath) {
-      const url = `${this.apiBase}/api/files/download?path=${encodeURIComponent(filePath)}`;
-      const link = document.createElement('a');
-      link.href = url;
-      link.setAttribute('download', '');
-      document.body.appendChild(link);
-      link.click();
-      document.body.removeChild(link);
+    // 本地存储下载（先预检，避免浏览器下载 JSON 错误文件）
+    async downloadFromLocal(filePath) {
+      try {
+        const checkResp = await axios.get(`${this.apiBase}/api/files/download-check`, {
+          params: { path: filePath }
+        });
+
+        if (!checkResp.data?.success) {
+          this.showToast('error', '下载失败', checkResp.data?.message || '下载失败，请稍后重试');
+          return false;
+        }
+
+        const url = `${this.apiBase}/api/files/download?path=${encodeURIComponent(filePath)}`;
+        const link = document.createElement('a');
+        link.href = url;
+        link.setAttribute('download', '');
+        document.body.appendChild(link);
+        link.click();
+        document.body.removeChild(link);
+        return true;
+      } catch (error) {
+        console.error('本地下载预检失败:', error);
+        const message = error.response?.data?.message || '下载失败，请稍后重试';
+        this.showToast('error', '下载失败', message);
+        if (error.response?.status === 401) {
+          this.logout();
+        }
+        return false;
+      }
     },
 
     // ===== 文件操作 =====